1. Field of the Invention
The present invention relates to a method of making a through hole in a copper-clad laminate having at least two copper layers with a carbon dioxide gas laser. More specifically, it relates to a method of making a through hole by directly irradiating a copper-clad laminate surface with the energy of a high-output carbon dioxide gas laser without the pre-removal of a surface copper foil by etching. A printed wiring board to which the above copper-clad laminate having the through hole made above is applied is used mainly for a small-sized semiconductor plastic package.
2. Prior Art of the Invention
In a high-density printed wiring board used for a semiconductor plastic package, etc., a through hole is conventionally made with a drill. In recent years, the through hole diameter is decreasing to 0.15 mm or less, and the diameter of the drill is therefore decreasing. When a hole having the above small diameter is made, the problem is that the drill bents or breaks or that the processing speed is low, due to the small diameter of the drill, which results in problems in productivity and reliability.
Further, in a high-density printed wiring board, the width and space of a circuit are sharply decreasing, and in some printed wiring boards, the line/space is 100 xcexcm/100 xcexcm or less. In this case, a pattern breakage or a short circuit often occurs, and low yields are caused.
Further, when there is employed a method in which holes having the same size are made in copper foils on upper and lower surfaces through negative films according to a predetermined method and a through hole reaching the upper and lower surfaces with a carbon dioxide gas laser, the problem is that the positions of the holes on the upper and lower surfaces deviate from each other so that it is difficult to form a land. Further, for making a hole through a copper foil on a reverse surface by irradiating the reverse surface with a carbon dioxide gas laser, a high-output energy is required. Therefore, the laser is confined to a metal sheet by placing the metal sheet on the reverse side. Even in this case, when the metal sheet has gloss, laser beam is reflected to hit the reverse surface of a copper-clad laminate, which causes a defect. Moreover, it has been impossible to make a hole in a plurality of copper-clad laminates at the same time.
A copper-clad laminate formed of a glass cloth substrate and a thermosetting resin has the following problems. When a through hole is made therein with a low-output carbon dioxide gas laser, it is difficult to process the glass and the hole wall is fluffed, or when the output of the carbon dioxide gas laser is high, the hole wall is not straightly formed and deformed. Further, when the copper foil is directly irradiated with a carbon dioxide gas laser, the laser is reflected and no hole can be made.
It is an object of the present invention to provide a method of making a small-diameter hole having high reliability with regard to a hole wall at a high rate with the energy of a high-output carbon dioxide gas laser without pre-making any hole in a copper foil.
It is another object of the present invention to provide a method of making small-diameter holes having high reliability with regard to a hole wall in a plurality of stacked copper-clad laminates at a high rate with the energy of a high-output carbon dioxide gas laser without pre-making any hole in copper foils.
It is further another object of the present invention to provide a copper-clad laminate suitable for making a hole with a high-output carbon dioxide gas laser.
It is still further another object of the present invention to provide an auxiliary material which enables the easy formation of a highly reliable hole with a carbon dioxide gas laser when brought into intimate contact to a copper foil surface to be directly irradiated with a carbon dioxide gas laser.
It is yet another object of the present invention to provide a backup sheet which enables the easy formation of a highly reliable hole with a carbon dioxide gas laser.
According to the present invention, there is provided a method of forming a penetration hole for a through hole in a thermosetting resin copper-clad laminate having at least two copper layers, in which copper foils of the thermosetting resin copper-clad laminate are processed with an energy of 20 to 60 mJ/pulse sufficient for removing the copper foils by means of the pulse oscillation of a carbon dioxide gas laser, the method comprising forming or disposing a coating or a sheet of an organic substance containing 3 to 97% by volume of at least one powder selected from the group consisting of a metal compound powder, a carbon powder and metal powder which have a melting point of at least 900xc2x0 C. and a bond energy of at least 300 KJ/mol on at least a copper foil surface to be irradiated with the carbon dioxide gas laser, and irradiating a surface thereof with necessary pulses of the carbon dioxide gas laser to form the penetration hole.
According to the present invention, there is also provided a method of making a penetration hole for a through hole according to the above, wherein, after the penetration hole is formed, both the copper foil surfaces are etched to remove part of the thickness thereof and form a smooth surface and at the same time to remove copper foil burrs fluffing on a penetration hole portion.
According to the present invention, there is also provided a method of making penetration holes according to the above, wherein coatings or sheets of an organic substance containing 3 to 97% by volume of at least one powder selected from the group consisting of a metal compound powder having a melting point of at least 900xc2x0 C. and a bond energy of at least 300 KJ/mol, a carbon powder and metal powder are formed or disposed on copper foil surfaces of 2 to 10 copper-clad laminates, one coating or sheet on the copper foil surface of one copper-clad laminate, the 2 to 10 copper-clad laminates are stacked, and the upper surface of the stacked copper-clad laminates is irradiated with the carbon dioxide gas laser to form the penetration holes at the same time.
According to the present invention, there is also provided a copper-clad laminate for use in a method of forming a penetration hole for a through hole in a thermosetting resin copper-clad laminate having at least two copper layers, in which copper foils of the thermosetting resin copper-clad laminate are processed with an energy of 20 to 60 mJ/pulse sufficient for removing the copper foils by means of the pulse oscillation of a carbon dioxide gas laser, the copper-clad laminate being a copper-clad laminate containing a prepreg of a glass fabric substrate which is impregnated with a resin composition containing a thermosetting resin having a glass transition temperature of at least 150xc2x0 C. and 10 to 60% by weight an insulating inorganic filler, the copper-clad laminate having a cross section wherein the thermosetting resin and the inorganic filler from the resin composition are homogeneously mixed.
According to the present invention, there is provided an auxiliary material which is for use on a copper foil surface of a copper-clad laminate when a penetration hole is made in the copper-clad laminate with a carbon dioxide gas laser by irradiating the copper foil surface with an energy of 20 to 60 mJ/pulse sufficient by means of the pulse oscillation of a carbon dioxide gas laser, and which is a coating or a sheet of an organic substance containing 3 to 97% by volume of at least one powder selected from the group consisting of a metal compound powder, a carbon powder and metal powder which have a melting point of at least 900xc2x0 C. and a bond energy of at least 300 KJ/mol.
According to the present invention, there is also provided a backup sheet for making a hole with a carbon dioxide gas laser, which is for use on a reverse outermost copper foil surface of a copper-clad laminate when a hole is made in the copper-clad laminate with a carbon dioxide gas laser by irradiating a front copper foil surface with an energy of 20 to 60 mJ/pulse by means of the pulse oscillation of a carbon dioxide gas laser and which comprises a 20 to 200 xcexcm thick resin layer and a metal plate.